Investigations have been continued on the impact upon arboviral cycles that will be produced by the profound ecological changes which are to occur in a tropical forest of eastern Panama by the construction of a hydroelectric dam across the Bayano river. Presently base-line pre-impoundment is being gathered on a number of parameters in order to assess the effect of the formation of the lake on them. Among these parameters, are physical environmental factors, population dynamics of possible vertebrate hosts and insect vectors and profiles of arboviral infection in wild populations of these vertebrates and insects as well as on sentinel animals including monkeys, sloths, hamsters and chickens. Target date for impoundment is now set for early summer 1975. Studies are also being conducted on the bionomics of Culex (Melanoconion) aikenii, vector of endemic VEE in two genetically distinct laboratory colonies. Surveillance for jungle yellow fever in eastern Panama continued. The epizootic wave detected near the Colombian border in 1971 moved as predicted toward the Panama Canal. It arrived in the Bayano river area in January, 1974 with possibly 16 human cases, and four deaths. One of this was confirmed by histophatological diagnosis and virus isolation.